1. Field of the Invention
The invention relates to a component consisting of polyvinyl acetate and alkali water glass for producing thermoplastically deformable, biodegradable and bright polymer mixtures based on starch, with good dimensional stability in water, which can be applied for the manufacture of sheet material, semi-finished and finished products, for example for packagings, containers and articles required in horticultural nurseries, in particular growing or cultivation aids, as well as in other fields of application. The invention, furthermore, relates to a process for producing said component.
2. The Prior Art
Numerous methods have become known in the last few years for producing and shaping thermoplastic starch (TPS) either alone or in the form of a polymer mixture or polymer melt or polymer blend (in the following referred to as a polymer mixture). Said methods were developed with the goal to open up new or expanded fields of application for regrowing raw materials. The basis of all inventions that have become known heretofore is the finding that the grainy structure of native starch can be changed into thermoplastic material first with defined proportions of water and/or lower polyfunctional alcohols such as ethylene glycol, propylene glycol, glycerol, 1,3-butane-diol, diglyceride, and the respective ethers, but also with compounds such as dimethyl sulfoxide, dimethyl formamide, dimethyl urea, dimethyl acetamide, and/or other additives, through thermomechanical breakdown with the help of conventional extruders; and that such materials then can be molded.
The useful value of the extrudate and of the products produced from the latter is low. In particular, the material is highly hydrophilic.
More recent proposals are dealing with the admixture of synthetically obtained waterproof polymers such as, for example polyethylene, polypropylene, polycaprolactone, as mixing components for starch. In this connection, however, the problem arises that the compatibility between the polymer components is inadequate, and that the biodegradability and also the cost structure become unfavorable.
The prior art is extensively documented in the relevant literature. Reference is made in this connection, for example to the publication by R. F. T. Stepto et al “Injection Molding of Natural Hydrophilic Polymers in the Presence of Water”, Chimia 41 (1987), No. 3, pp. 76-81, and the literature cited there, as well as, for example to patents DE 4116404; EP 0327505; DE 4038732; U.S. Pat. No. 5,106,890; DE 4117628; WO 94/04600; DE 4209095; DE 4122212, EP 0404723; and EP 407350.
In DE 40 38 732, the starch plasticized with water and glycerol is processed in the extruder to a polymer mixture predominantly with polyvinyl acetate. The extrudate exhibits superior resistance to water as compared to TPS. With higher amounts of starch components, the extrudate or the bottles produced therefrom take on a yellowish to brownish color. This limits the starch component to below 50%.
A slightly acid to neutral component consisting of polyvinyl acetate and water glass has already been proposed (DE 195 33 800) in which a polymer mixture of starch and a hydrophobic polymer, e.g. polyvinyl acetate, can be extruded. The component is produced from water glass and polyvinyl acetate as well as, if need be, further acid components for adjusting the pH, by the extrusion process with intensive mixing. It has been found that even only minor additions of said component lead to a significant qualitative improvement of the extrudate and of the products manufactured from the latter. As compared to the prior art, native starch can be used in considerably greater amounts without or with only minor discoloration and while maintaining or enhancing the dimensional stability in water. The component obviously contributes to the fact that the two phases, which are not compatible in mixture per se, render the hydrophilic thermoplastic starch and the hydrophobic polymer miscible to a certain degree.
Further tests have shown that the quality of the final products has to be improved even further with respect to dimensional stability in water and strength. In particular, it has not been possible to manufacture thin sheet materials with a thickness of below about 300 μm.